The underestimated importance of belowground carbon input for forest soil animal food webs

The present study investigated the relative importance of leaf and root carbon input for soil invertebrates. Experimental plots were established at the Swiss Canopy Crane (SCC) site where the forest canopy was enriched with C-13 depleted CO2 at a target CO2 concentration of c. 540 p.p.m. We exchanged litter between labelled and unlabelled areas resulting in four treatments: (i) leaf litter and roots labelled, ( ii) only leaf litter labelled, ( iii) only roots labelled and ( iv) unlabelled controls. In plots with only C-13-labelled roots most of the soil invertebrates studied were significantly depleted in C-13, e.g. earthworms, chilopods, gastropods, diplurans, collembolans, mites and isopods, indicating that these taxa predominantly obtain their carbon from belowground input. In plots with only C-13-labelled leaf litter only three taxa, including, e.g. juvenile Glomeris spp. (Diplopoda), were significantly depleted in C-13 suggesting that the majority of soil invertebrates obtain its carbon from roots. This is in stark contrast to the view that decomposer food webs are based on litter input from aboveground.